added rotate_p

docs/lib-rotate_p.md

@@ -0,0 +1,57 @@
+# rotate_p
+
+Rotates a point `a` degrees around an arbitrary axis. The rotation is applied in the following order: `x`, `y`, `z`. 
+
+## Parameters
+
+- `point` : The point `[x, y, z]`.
+- `a` : An array `[deg_x, deg_y, deg_z]`. The same as the `a` parameter of the built-in `rotate`.
+
+## Examples
+    
+You can use the code below to create a line.
+
+	hull() {
+	    sphere(1);
+	    rotate([0, -45, 45]) 
+	        translate([20, 0, 0]) 
+	            sphere(1);   
+	}  
+
+The following code have the same effect.
+
+	point = [20, 0, 0];
+	a = [0, -45, 45];
+	
+	hull() {
+	    sphere(1);
+	    translate(rotate_p(point, a))    
+	       rotate(a)  
+	           sphere(1);   
+	}  
+
+![rotate_p](images/lib-rotate_p-1.JPG)
+
+The `rotate_p` function is useful in some situations. For examples, you probably want to get all points on the path of a spiral around a sphere. 
+
+	radius = 40;
+	step_angle = 10;
+	z_circles = 20;
+	
+	points_angles = [for(a = [0:step_angle:90 * z_circles]) 
+	    rotate_p(
+	        [radius, 0, 0], 
+	        [0, -90 + 2 * a / z_circles, a]
+	    )
+	];
+	
+    // Once you get all points on the path, you can place anything at each point.
+    // I just place a sphere as a simple demonstration.
+	for(pa = points_angles) {
+	    translate(pa[0]) 
+	        sphere(1);
+	}
+
+    %sphere(radius);
+
+![rotate_p](images/lib-rotate_p-2.JPG)